With the growing demands for more powerful electric tools, battery packs structured as the battery power source of such tools are required to have a large power output capacity. Battery packs composed of a large number of rechargeable batteries accommodated in a pack case have been used to satisfy this requirement. However, as the electric tools are hand-held when operated, smaller and lighter battery packs are sought after. Nickel cadmium rechargeable batteries or nickel metal hydride rechargeable batteries are generally used for the battery packs. But these are not sufficiently satisfactory in respect of weight energy density and volume energy density, and had the problem of poor operability of the electric tool because of the large and heavy battery pack that requires a large number of rechargeable batteries, and the problem of poor heat dissipation because of the multiple, densely packed batteries.
Non-aqueous electrolyte rechargeable batteries such as lithium ion rechargeable batteries are known, which have superior weight and volume energy density as compared to the above-mentioned nickel cadmium rechargeable batteries or nickel metal hydride rechargeable batteries. By using such batteries, the battery pack can be made smaller and lighter with the same output power. Flat prismatic lithium ion rechargeable batteries, in particular, can more advantageously be used than cylindrical batteries to make a battery pack with superior space efficiency.
However, because rechargeable batteries with non-aqueous electrolyte such as lithium ion rechargeable batteries tend to deteriorate or generate heat when subjected to overcharge and overdischarge, they need a battery protection circuit and/or charge/discharge control circuit. Therefore a battery pack is usually formed by accommodating a circuit substrate with these circuits formed thereon together with the rechargeable batteries within a pack case. Many design proposals have been made for such battery packs with rechargeable batteries and a circuit substrate accommodated inside a pack case, and to give an example, a battery pack shown in the following patent document 1 is known.
Document 1: Japanese Patent Laid-Open Publication No. 2001-313015.
However, with a battery pack that is used for a relatively high-power application such as a battery power source of an electric tool, because of the use of a large number of rechargeable batteries, there was a problem that the wiring structure for connecting each one of the rechargeable batteries to the circuit substrate was complex. Moreover, electric tools are often used in an environment where they are exposed to dust or water and for that reason they need a structure for preventing entrance of water, moisture, and dust into electrical circuit parts such as a circuit substrate. But further requirements are that the heat dissipation properties of the rechargeable batteries must not be deteriorated and that the battery pack must not be oversized because of such protective structure.
Accordingly, an object of the present invention is to provide a battery pack having a simple connection structure for integrating a circuit substrate with a plurality of rechargeable batteries and a structure for preventing entrance of water, moisture, and dust into electrical circuit parts.